Filter

ABSTRACT

A filter includes first and second line patterns each having a length substantially equal to ½ of the wavelength of a pass-band frequency, and a resonator that is interposed between the first and second line patterns and is coupled therewith so that the first and second line patterns have open stubs in which connection points between input/output terminals and the first and second line patterns appear to be short-circuited when viewed from ends of the first and second line patterns.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a filter, and moreparticularly, to a filter that is used in high-frequency ranges havingwavelengths of, for example, microwaves, sub-millimeters or millimeters.

2. Description of the Related Art

In general, filters used in high-frequency ranges are formed withdistributed constant circuits, which may, for example, includemicrostrip lines or coplanar lines. Filters using microstrip lines aredisclosed in Japanese Patent Application Publication No. 2002-026605 and“Low Cost Planar Filter for 60 GHz Applications (Yoshihisa Amano, etal., 30^(th) European Microwave Conference in Paris 2000, pp. 340-343)”.In each of those filters, two λ/2 open-line resonators (λ being thewavelength of an electric signal propagating through the line in thevicinity of the center frequency of the pass band) are connected throughcapacitive coupling by an electromagnetic coupler, and an input terminaland an output terminal are connected through mutually inductive couplingby an electromagnetic coupler. With this structure, the frequencies ofthe attenuation poles can approach the center frequency, and the cut-offprofile of the filter frequency can become sharper.

With the prior art disclosed in Japanese Patent Application PublicationNo. 2002-026605, however, the patterns are too complicated to reduce thesize of the filter, and only a low degree of freedom is allowed in thestage of designing the filter.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a filterin which the above disadvantage is eliminated.

A more specific object of the present invention is to provide a filterwith a simpler structure and a higher degree of freedom in design.

The above objects of the present invention are achieved by a filterincludes first and second line patterns each having a lengthsubstantially equal to ½ of the wavelength of a pass-band frequency, anda resonator that is interposed between the first and second linepatterns and is coupled therewith so that the first and second linepatterns have open stubs in which connection points between input/outputterminals and the first and second line patterns appear to beshort-circuited when viewed from ends of the first and second linepatterns.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a filter as a comparative example of the presentinvention;

FIG. 2 illustrates the principles of the present invention;

FIG. 3 illustrates a filter in accordance with a first embodiment of thepresent invention;

FIG. 4 is a graph showing the frequency characteristics of the firstembodiment shown in FIG. 3;

FIG. 5 illustrates a modification of the first embodiment shown in FIG.3;

FIG. 6 is a graph showing the frequency characteristics of themodification shown in FIG. 5;

FIGS. 7A through 7D each illustrate a filter in accordance with a secondembodiment of the present invention;

FIGS. 8A through 8C each illustrate a filter in accordance with a thirdembodiment of the present invention; and

FIG. 9 illustrates a filter in accordance with a fourth embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

So as to solve the above-described problem, the inventors used stubs ina filter. FIG. 1 illustrates the structure of a filter that employsstubs. The filter shown in FIG. 1 was developed by the inventors in thecourse of inventing the present invention, and functions as a bandpassfilter. In the following, the filter shown in FIG. 1 will be referred toas a comparative example.

The filter shown in FIG. 1 includes microstrip lines 1 and 2, resonators5 and 6, and input/output terminals 7 and 8. These patterns are formedon a dielectric substrate 9. Stub portions 3 and 4 are formed on themicrostrip lines 1 and 2, respectively. Each of the microstrip lines 1and 2 has a length of λ/2 (λ being the wavelength of an electric signalpropagating through the transmission path at the center frequency of thepass band or a frequency close thereto; hereinafter the wavelength λwill be also referred to as the ½ wavelength of the pass-bandfrequency), and is able to efficiently input and output power. Theinput/output terminals 7 and 8, the microstrip lines 1 and 2, and theresonators 5 and 6 are electromagnetically coupled so as to set the passband in the vicinity of the resonance frequency.

The stub portions 3 and 4 function as open stubs, and are designed sothat the connection points between the input/output terminals 7 and 8and the stub portions 3 and 4 appear to be short-circuited when viewedfrom the ends of the stub portions 3 and 4. The length of each of thestub portions 3 and 4 is λ/4, for example. As the stub portions 3 and 4function as open stubs, attenuation poles are formed in the vicinity ofthe resonance frequency. It is essential to form attenuation poles inthe vicinity of the resonance frequency in achieving excellent filtercharacteristics with a sharp cut-off profile.

However, the inventor founds out that the filter shown in FIG. 1 has adifficulty in achieving filter characteristics with a sharp cut-offprofile. As described above, to achieve desirable filtercharacteristics, it is necessary to form attenuation poles near the passband. Taking other conditions into consideration, however, theattenuation poles might overlap the pass band or appear far away fromthe pass band. The attenuation poles might not even appear at all. Anelectromagnetic simulator, which is used to design the filter, is toobtain approximate values with respect to the electromagneticdistribution, and therefore, cannot exhibit desired characteristics whenthe filter is put into practical use, though ideal characteristics canbe obtained in the stage of designing. In this filter, the probabilityof obtaining desired attenuation poles can be increased by increasingthe number of elements that form attenuation poles, i.e., increasing thenumber of stubs, or selecting stubs with desired characteristics.Accordingly, more freedom can be allowed in the stage of designing.However, an increased number of stubs requires more area for the addedstubs, resulting in an increase in size.

Based on the above observations, the inventors have developed a filterwith a simpler structure and more freedom of design.

FIG. 2 illustrates a filter that employs microstrip lines as linepatterns in accordance with the present invention. The filter shown inFIG. 2 includes a dielectric substrate 17, and has a first microstripline 11, a second microstrip line 12, a resonator 13, and input/outputterminals 15 and 16, all of which are formed on the dielectric substrate17. Each of the first microstrip line 11 and the second microstrip line12 has a length substantially equal to ½ of the wavelength λcorresponding to the center frequency of the pass band or close thereto.The resonator 13 is interposed between the first microstrip line 11 andthe second microstrip line 12. The resonator 13 and the first microstripline 11 are coupled so that the first strip line 11 includes open stubsin which the connection point between the input/output terminal 15 andthe first microstrip line 11 appears to be short-circuited when seenfrom ends 14 of the first microstrip line 11. The resonator 13 and thesecond microstrip line 12 are coupled so that the second microstrip line12 has open stubs in which the connection point between the input/outputterminal 16 and the second microstrip line 12 appears to beshort-circuited when seen from ends 14 of the-second microstrip line 12.As a result, four open stubs that can be expected to contribute toexcitation can be obtained. The four open stubs may not have anattenuation pole or a clear attenuation pole, contribute to forming anattenuation characteristic. In accordance with the present invention,two open stubs are formed on each strip line at the input/output sides.Thus, a large number of stubs can be obtained for the size, and a higherdegree of freedom can be allowed for design.

Although four open stubs can be obtained in accordance with the presentinvention, the number of attenuation poles might be less than four evenwhen the present invention is employed. This is because the frequenciesof the attenuation poles of the open stubs might be close to or overlapone another, or might not appear at all under certain conditions. Still,the advantage of having many controllable open stubs is maintained insuch cases.

In the above example, microstrip lines are employed as line patterns.However, the present invention can be embodied by employing othertransmission lines such as coplanar lines.

The following is a description of embodiments of the present invention,with reference to the accompanying drawings.

(First Embodiment)

FIG. 3 illustrates a filter in accordance with a first embodiment of thepresent invention. The filter shown in FIG. 3 includes four microstriplines 21, 22, 23, and 24, and two input/output terminals 25 and 26.These patterns are formed on a dielectric substrate having a groundpattern formed on the back. Each of the microstrip lines 21 through 24is an open-looped transmission path that is substantially equivalent to½ of the wavelength λ corresponding to the center frequency of the passband or a frequency in the neighborhood of the center frequency. So asto form an open loop, each of the microstrip lines 21 through 24 hasfour bent portions. Having loop-like forms, the microstrip lines 21through 24 can be arranged in a relatively small area. The microstriplines 21 and 22 are arranged to provide hybrid coupling by combiningcapacitive coupling and inductive coupling, the microstrip lines 22 and24 are arranged to provide inductive coupling, and the microstrip lines24 and 23 are arranged to provide hybrid coupling by combiningcapacitive coupling and inductive coupling. Each of the microstrip lines22 and 24 forms a resonator. Accordingly, the filter shown in FIG. 3 hasresonators coupled to each other.

The input/output terminal 25 is provided for the microstrip line 21, andthe input/output terminal 26 is provided for the microstrip line 23. Themicrostrip line 22 is coupled to the microstrip line 21 so that themicrostrip line 21 has open stubs in which the connection point betweenthe input/output terminal 25 and the microstrip line 21 appears to beshort-circuited when seen from ends 27 of the microstrip line 21.Likewise, the microstrip line 24 is coupled to the microstrip line 23 sothat the microstrip line 23 has open stubs in which the connection pointbetween the input/output terminal 26 and the microstrip line 23 appearsto be short-circuited when seen from ends 28 of the microstrip line 23.The input/output terminal 25 is located on the side of the microstripline 21 opposite to the side on which the microstrip line 22 is located.Likewise, the input/output terminal 26 is located on the side of themicrostrip line 23 opposite to the side on which the microstrip line 24is located. Accordingly, the input/output terminals 25 and 26 extend inthe same direction as each other.

FIG. 4 shows the frequency characteristics of the filter shown in FIG.3. In FIG. 4, the horizontal axis indicates frequency (GHz), and thevertical axis indicates attenuation (dB). The solid-line curve indicatespass characteristics S21, and the dotted-line curve indicates reflectioncharacteristics S11. An attenuation pole P1 is formed in the vicinity ofthe low frequency side of the pass band, and the attenuation at theattenuation pole P1 is approximately −37 dB. With the center frequency fof the pass band being 1, the location of the attenuation pole P1 isapproximately 0.87 f. Accordingly, the cut-off profile of the lowfrequency side of the pass band is very sharp. An attenuation pole P2with a smaller attenuation than the attenuation pole P1 is formed on thehigh frequency side of the pass band. The filter of this embodimentexhibits asymmetric filter characteristics in terms of attenuation, butcan function as a band-pass filter.

FIG. 5 is a modification of the first embodiment. The filter shown inFIG. 5 also includes the four microstrip lines 21 through 24, but hasthe input/output terminals 25 and 26 located in different positions fromthe input/output terminals 25 and 26 of the first embodiment. Theinput/output terminal 25 shown in FIG. 5 is located closer to themicrostrip line 22 than to the line that divides the microstrip line 21into two equal parts. Likewise, the input/output terminal 26 is locatedcloser to the microstrip 24 than to the line that divides the microstripline 23 into two equal parts. In the structure shown in FIG. 5, themicrostrip line 22 is also coupled to the microstrip line 21 so that themicrostrip line 21 has open stubs in which the connection point betweenthe input/output terminal 25 and the microstrip line 21 appears to beshort-circuited when seen from the ends 27 of the microstrip line 21.Likewise, the microstrip line 24 forms a resonator and is coupled to themicrostrip line 23 so that the microstrip line 23 has open stubs inwhich the connection point between the input/output terminal 26 and themicrostrip line 23 appears to be short-circuited when seen from the ends28 of the microstrip line 23.

FIG. 6 shows the frequency characteristics of the filter shown in FIG.5. In FIG. 6, the horizontal axis indicates frequency (GHz), and thevertical axis indicates attenuation (dB). The solid-line curve indicatespass characteristics S21, and the dotted-line curve indicates reflectioncharacteristics S11. An attenuation pole P2 is formed in the vicinity ofthe high frequency side of the pass band, and the attenuation at theattenuation pole P2 is approximately −44 dB. With the center frequency fof the pass band being 1, for example, the location of the attenuationpole P2 is approximately 1.12 f. Accordingly, the cut-off profile on thehigh frequency side of the pass band is very sharp. An attenuation poleP1 with a smaller attenuation than the attenuation pole P2 is formed onthe low frequency side of the pass band. The filter of this embodimentexhibits asymmetric filter characteristics in terms of attenuation, butcan function as a band-pass filter.

The structures shown in FIGS. 4 and 6 differ from each other in thecoupling of the microstrip lines 22 and 24 to the microstrip lines 21and 23, respectively. This implies that the attenuation poles can beadjusted by adjusting the positions of the input/output terminals 25 and26, and a higher freedom is allowed for design.

(Second Embodiment)

FIGS. 7A through 7D each illustrate a filter in accordance with a secondembodiment of the present invention. The filters shown in FIGS. 7Athrough 7D use microstrip lines as transmission paths but have differentresonator structures from one another. Each of the filters includesmicrostrip lines 31 and 32 of λ/2 in length, and input/output terminals34 and 35 that are provided for the microstrip lines 31 and 32,respectively. The microstrip lines 31 and 32 and the input/outputterminals 34 and 35 are formed on a dielectric substrate that isindicated by a broken line in each of FIGS. 7A through 7D. A resonatorthat is described below is interposed between the microstrip lines 31and 32 in each of the filters. The resonator is coupled to themicrostrip line 31 so that the microstrip line 31 has open stubs inwhich the connection point between the input/output terminal 34 and themicrostrip line 31 appears to be short-circuited when seen from the endsof the microstrip line 31. The resonator is also coupled to themicrostrip line 32 so that the microstrip line 32 has open stubs inwhich the connection point between the input/output terminal 35 and themicrostrip line 32 appears to be short-circuited when seen from the endsof the microstrip line 32. In the examples shown in FIGS. 7A through 7D,the input/output terminal 34 is located slightly above the line thatdivides the microstrip line 31 into two equal parts, and theinput/output terminal 35 is located slightly below the line that dividesthe microstrip line 32 into two equal parts. Accordingly, theinput/output terminals 31 and 32 slightly deviate from each other andextend in the opposite directions from each other.

The filter shown in FIG. 7A has a λ/2 open-line resonator 33A. Thefilter shown in FIG. 7B has a capacity-loaded λ/2 open-line resonator33B. The filter shown in FIG. 7C has a bent λ/2 open-line resonator 33C.The filter shown in FIG. 7D has a ring-type resonator 33D with acircumference of λ. Each of these filters exhibits the same frequencycharacteristics as the frequency characteristics shown in FIG. 4 or 6.

(Third Embodiment)

FIGS. 8A through 8C each illustrate a filter in accordance with a thirdembodiment of the present invention. The filters shown in FIGS. 8Athrough 8C use coplanar lines as transmission lines but have differentresonator structures from one another. Each of the filters includes linepatterns 41 and 42 of λ/2 in length, and input/output terminals 44 and45 that are provided for the line patterns 41 and 42, respectively. Theline patterns 41 and 42 and the input/output terminals 44 and 45 areformed on a dielectric substrate that is indicated by a broken line ineach of FIGS. 8A through 8C. A ground pattern 46 is also provided so asto surround both ends of each of the line patterns 41 and 42, therebyforming a coplanar line structure. A resonator that is described belowis interposed between the line patterns 41 and 42 in each of thefilters. The resonator is coupled to the line pattern 41 so that theline pattern 41 has open stubs in which the connection point between theinput/output terminal 44 and the line pattern 41 appears to beshort-circuited when seen from the ends of the line pattern 41. Theresonator is also coupled to the line pattern 42 so that the linepattern 42 has open stubs in which the connection point between theinput/output terminal 45 and the line pattern 42 appears to beshort-circuited when seen from the ends of the line pattern 42.

The filter shown in FIG. 8A has a λ/4 single-end open-line resonator43A. The filter shown in FIG. 8B has a λ/2 line resonator 43B havingboth of the end portions short-circuited. Both of the end portions ofthe resonator 43B are connected to the ground pattern 46. The filtershown in FIG. 8C has a λ/2 open-line resonator 43C having both of theend portions left open. Each of these filters exhibits the samefrequency characteristics as the frequency characteristics shown in FIG.4 or 6.

(Fourth Embodiment)

FIG. 9 illustrates a filter in accordance with a fourth embodiment ofthe present invention. The filter shown in FIG. 9 is the same as thefilter shown in FIG. 7D, except that the ring-type resonator 33D isreplaced with a dielectric resonator 53. The other aspects of thestructure of the filter shown in FIG. 9 are the same as those of thestructure of the filter shown in FIG. 7D. The dielectric resonator 53 iscoupled to the microstrip line 31 so that the microstrip line 31 hasopen stubs in which the connection point between the input/outputterminal 34 and the microstrip line 31 appears to be short-circuitedwhen seen from the ends of the line pattern 31. The dielectric resonator53 is also coupled to the microstrip line 32 so that the microstrip line32 has open stubs in which the connection point between the input/outputterminal 35 and the microstrip line 32 appears to be short-circuitedwhen seen from the ends of the microstrip line 32. The filter shown inFIG. 9 exhibits the same frequency characteristics as the frequencycharacteristics shown in FIG. 4 or 6.

The present invention also provides filters that have resonators thathave different patterns from the resonators described above, or usedifferent line patterns (such as suspended lines or slot lines) from theline patterns described above.

The filters shown in FIG. 3 and the filter shown in FIG. 5 may beconnected in series, so as to form a filter that has the combinedfrequency characteristics of those shown in FIGS. 4 and 6. In such acase, the cut-off profiles on the low frequency side and the highfrequency side of the pass band become sharper by virtue of theattenuation poles formed on both ends of each line pattern that functionas open stubs.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A filter comprising: first and second line patterns each having alength substantially equal to ½ of the wavelength of a pass-bandfrequency; and a resonator that is interposed between the first andsecond line patterns and is coupled therewith so that the first andsecond line patterns have open stubs in which connection points betweeninput/output terminals and the first and second line patterns appear tobe short-circuited when viewed from ends of the first and second linepatterns.
 2. The filter as claimed in claim 1, wherein the open stubsform attenuation characteristics or attenuation poles.
 3. The filter asclaimed in claim 1, wherein the open stubs form two or more attenuationpoles.
 4. The filter as claimed in claim 1, wherein the resonatorincludes a plurality of resonators coupled in turn.
 5. The filter asclaimed in claim 1, wherein the first and second line patterns have abent pattern or a loop pattern.
 6. The filter as claimed in claim 1,wherein the first and second line patterns are microstrip lines.
 7. Thefilter as claimed in claim 1, wherein the first and second line patternsare coplanar lines.
 8. The filter as claimed in claim 1, wherein theinput/output terminals are located at positions that deviate fromcenters of the first and second line patterns.